Flat panel, color displays for displaying information, including images, text, and graphics are widely used. These displays may employ any number of known technologies, including liquid crystal light modulators, plasma emission, electro-luminescence (including organic light-emitting diodes), and field emission. Such displays include entertainment devices such as televisions, monitors for interacting with computers, and displays employed in hand-held electronic devices such as cell phones, game consoles, and personal digital assistants. In these displays, the resolution of the display is always a critical element in the performance and usefulness of the display. The resolution of the display specifies the quantity of information that can be usefully shown on the display and the quantity of information directly impacts the usefulness of the electronic devices that employ the display.
The term “resolution” is often used or misused to represent any number of quantities. Common misuses of the term include referring to the number of light-emitting elements or to the number of full-color groupings of light-emitting elements (typically referred to as pixels) as the “resolution” of the display. This number of light-emitting elements is more appropriately referred to as the addessability of the display. Within this document, we will use the term “addressability” to refer to the number of light-emitting elements per unit area of the display device. A more appropriate definition of resolution is to define the size of the smallest element that can be displayed with fidelity on the display. One method of measuring this quantity is to display the narrowest possible, neutral (e.g., white) horizontal or vertical line on a display and to measure the width of this line or to display an alternating array of neutral and black lines on a display and to measure the period of this alternating pattern. Note that using these definitions, as the number of light-emitting elements increases within a given display area, the addressability of the display will increase while the resolution, using this definition, generally decreases. Therefore, counter to the common use of the term “resolution”, the quality of the display is generally improved as the resolution becomes finer in pitch or smaller.
The term “apparent resolution” refers to the perceived resolution of the display as viewed by the user. Although methods for measuring the physical resolution of the display device are typically designed to correlate with apparent resolution, it is important to note that this does not always occur. At least two important conditions exist under which the physical measurement of the display device does not correlate with apparent resolution. The first of these occurs when the physical resolution of the display device is small enough that the human visual system is unable to resolve changes in physical resolution (i.e., the apparent resolution of the display becomes eye-limited). The second condition occurs when the measurement of the physical resolution of the display is performed for only the luminance channel but not performed for resolution of the color information while the display actually has a different resolution within each color channel, therefore overstating the apparent resolution for the color channels.
Addressability in most flat-panel displays, especially active-matrix displays, is limited by the need to provide signal busses and electronic control elements in the display. Further in many flat panel displays, including Liquid Crystal Displays (LCDs) and bottom-emitting Electro-Luminescent (EL) displays, the electronic control elements are required to share the area that is required for light emission or transmission. In these technologies, as the area required to constitute the busses and control elements increases, the proportion of the display area that is available for actual light-emitting decreases. Depending upon the technology, reduction of the area of the light-emitting area can reduce the efficiency of light output, as is the case for LCDs, or reduce the brightness and/or lifetime of the display device, as is the case for EL displays. Regardless of whether the area required for patterning busses and control elements compete with the light-emitting area of the display, the decrease in buss and control element size that occur with increases in addressability for a given display generally require more accurate, and therefore more complex, manufacturing processes and can result in greater number of defective panels, decreasing yield rate and increasing the cost of marketable displays. Therefore, from a cost and manufacturing complexity point of view, it is generally advantageous to be able to provide a display with lower addressability. This desire is, of course, in conflict with the need to provide higher apparent resolution. Therefore, it is desirable to provide a display with relatively low addessability but high apparent resolution.
It has been known for many years that the human eye is more sensitive to spatial detail when it is presented using variations in luminance then when presented using variations in chrominance information. In the field of electronic displays, full-color displays typically employ red, green, and blue light-emitting elements. In these displays, while the red and blue light-emitting elements are necessary to form a full-color display, they often provide much less luminance than the green light-emitting elements. Therefore, it is known to employ a larger number of high-luminance green light-emitting elements than red or blue. Takashi et al. in U.S. Pat. No. 5,113,274, entitled “Matrix-type color liquid crystal display device”, has proposed the use of displays having two green for every red and blue light-emitting element. Further, the introduction of additional high-luminance light-emitting elements that provide other colors of light-emission can have positive effects beyond providing higher perceived quality. For example, within the field of Organic Light Emitting Diodes (OLEDs), it is known to introduce more than three light-emitting elements where the additional light-emitting elements have higher luminance efficiency, resulting in a display having higher luminance efficiency. Such displays have been discussed by Miller et al. in U.S. Patent Application Publication 2004/0113875, entitled “Color OLED display with improved power efficiency” and in U.S. patent Application Publication 2005/0212728 also entitled “Color OLED display with improved power efficiency”.
The introduction of additional high-luminance light-emitting elements has been used in a variety of ways to optimize the frequency response of imaging systems. For example, relative sensitivities of the human eye to different color channels have recently been used in the liquid crystal display (LCD) art to produce displays having subpixels with broad band emission to increase perceived resolution. For example, U.S. Patent Application 2005/0225574 and U.S. Patent Application 2005/0225575, each entitled “Novel subpixel layouts and arrangements for high brightness displays” provide various subpixel arrangements that include a high-luminance (often white or cyan) subpixel that allows more of the white light generated by the LCD backlight to be transmitted to the user than the traditional filtered RGB subpixels. The subpixel arrangements discussed include ones in which each row and each pair of columns contain all colors of subpixels, making it possible to produce a line of any color using only one row or two columns of subpixels. Therefore, if the LCD is driven correctly, it can be argued that the vertical resolution of the device is equal to the height of one row of subpixels and the horizontal resolution of the device is equal to the width of two columns of subpixels, even though it requires more subpixels than the two subpixels at the intersection of such horizontal and vertical lines to produce a full-color image. It is important to note that in arrangements of light-emitting elements such as these, there are more high-luminance light-emitting elements than there are repeating patterns of light-emitting elements that are capable of producing a full-color image. Therefore, arrangements of light-emitting elements such as these allow a luminance pattern to be displayed with a higher spatial frequency than would be possible if each luminance signal was to be rendered to each repeating pattern of light-emitting elements. However, to achieve this goal, a proper rendering algorithm must be provided to provide this higher resolution rendering without creating significant color artifacts.
Many input image signals may be used to encode and transmit a full-color image for display. For example, an input image may be described in common RGB color spaces such as sRGB or in luminance/chrominance spaces such as YUV, L*a*b*, or YIQ. In any case, the input display signal must be converted to a signal suitable for driving the native display light-emitting elements. This conversion may involve steps such as conversion of a three-color input image signal to a signal to drive an array of four or more colors of light-emitting elements as described in U.S. Pat. No. 6,897,876 issued May 24, 2005 which are capable of achieving maximum display efficiency while providing accurate color. This conversion may also comprise methods such as subpixel interpolation like those described in U.S. Patent Application 2005/0225563, entitled “Subpixel rendering filters for high brightness subpixel layouts”, which allows an input image signal that is intended for display on an arrangement of subpixels to be interpolated such that the input data is more appropriately matched to an alternate arrangement of subpixels. While subpixel interpolation methods known in the art allow different spatial filtering operations to be performed on signals that are intended for display on subpixels having different colors, they do not fully allow the optimization of the signal to take advantage of the difference in the human visual system's sensitivities to luminance and chrominance information. In fact, these interpolation methods typically include a filtering process that blurs the high frequency information to render the image without significant color artifacts.
It is known in the art to perform separate processing steps on luminance than on chrominance-encoded signals. For example, U.S. Pat. No. 5,987,169, entitled “Method for improving text resolution in images with reduced chromatic bandwidth” recognizes that some compression means provide excessive blurring to high spatial frequency, high luminance chrominance information, resulting in text or other high spatial frequency image objects that appear blurred. To overcome this problem, this patent discusses reducing the chrominance signal for highly chromatic text displayed on bright (white) backgrounds.
U.S. Patent Application 2002/0154152, entitled “Display apparatus, display method and display apparatus controller” describes a display having red, green, and blue elements or subpixels which form full color pixels. This display receives an input image signal, converts the signal to a luminance and chrominance signal, then renders the luminance information to the subpixel level but renders the chrominance information to the pixel level, thus the luminance signal is represented at a higher spatial frequency than the chrominance signal, thereby providing a higher perceived resolution without visible lower frequency chromatic artifacts. It should be noted that for optimal performance the input image signal should address a number of spatial locations equal to the number of subpixels in the display device. However, because the arrangements of light-emitting elements that are discussed include only one high luminance light-emitting element per pixel and the low luminance red and blue elements provide only a low luminance signal the subpixel arrangement limits the usefulness of this approach. Further, this patent applies only linear transforms to convert from one three channel image representation to a second three-channel representation and as such can not be applied when converting an input three color signal to a four or more output color signal. Finally, the method ignores the fact that different tradeoffs between localized luminance and chrominance error may be made depending upon the spatial content of the image.
U.S. Pat. No. 6,507,350 entitled “Flat-panel display drive using sub-sampled YCBCR color signals” also discusses encoding an input three-color RGB signal into a luminance and chrominance color space and then later rendering the signal to a three-color RGB pixel pattern. This disclosure discusses the fact that the chrominance signal can be sub-sampled, reducing the bandwidth required to transmit the signal without visible artifacts. Once again, because the arrangements of light-emitting elements that are discussed include only one high luminance light-emitting element per pixel and the low luminance red and blue elements provide only a low luminance signal the subpixel arrangement limits the usefulness of this approach. Further, this patent applies only linear transforms to convert from one three channel image representation to a second three-channel representation and as such can not be applied when converting an input three color signal to a four or more output color signal. Finally, the method ignores the fact that different tradeoffs between localized luminance and chrominance error may be made depending upon the spatial content of the image.
U.S. Pat. No. 5,793,885 entitled “Computationally efficient low artifact system for spatially filtering digital color images” also discusses converting an input image to a luminance and chrominance domain and then applying sharpening to only the luminance channel in the input RGB image. By applying this processing step to the luminance channel, the image may be sharpened using a single convolution to the luminance channel rather than convolving each of the red, green, and blue image signals by separate sharpening kernels. Using this approach, the efficiency of the image processing system is improved. While this process sharpens the luminance channel within the image, it does not necessarily improve the reconstruction of edge information. Further, this patent applies only linear transforms to convert from one three channel image representation to a second three-channel representation and as such can not be applied when converting an input three color signal to a four or more output color signal. Further, it does not anticipate that such a method might be significantly more beneficial when provided in a display having more high-luminance subpixels than pixels or when applied in a display system having not only red, green, and blue light-emitting elements, but also additional light-emitting elements.
There is a need, therefore, for a display system with improved apparent resolution of a display device. Particularly, such a system should provide a means of providing a higher image quality when rendering an image to an arrangement of red, green, blue, and at least one additional high luminance light-emitting element. Further, it is desirable for such a system to consider the relative efficiency of the light-emitting elements to co-optimize the efficiency of the display device.